Chromate-free conversion layer and process for producing the same

ABSTRACT

A chromium(VI)-free, chromium(III)-containing and substantially coherent conversion layer on zinc or zinc alloys presenting, even in the absence of further components such as silicate, cerium, aluminum and borate, a corrosion protection of approx. 100 to 1000 h in the salt spray test according to DIN 50021 SS or ASTM B 117-73 until first attack according to DIN 50961 Chapter 10; being clear, transparent and substantially colorless and presenting multi-colored iridescence; having a layer thickness of approx. 100 nm to 1000 nm; and being hard, adhering well and being resistant to wiping.

FIELD OF THE INVENTION

The present invention relates to chromium(VI)-free,chromium(III)-containing, substantially coherent conversion layers, amethod for producing them, a concentrate, a passivation bath, apassivating method, a passive layer, and a conversion layer.

BACKGROUND OF THE INVENTION

Metallic materials, in particular iron and steel, are plated with zincor cadmium in order to protect them from corrosive environmentalinfluences. The corrosion protection of zinc resides in the fact that itis even less precious than the base metal and therefore at firstexclusively draws the corrosive attack; it acts as a sacrificial layer.The base metal of the respective zinc-plated component remainsunimpaired as long as it is continuously covered with zinc, and themechanical functionality remains preserved over longer periods of timethan in the case of parts not plated with zinc. Thicker zinc layersnaturally afford higher corrosion protection than thin layers inasmuchas corrosive erosion of thicker layers simply takes more time.

The corrosive attack on the zinc layer, in turn, can be greatly delayedby application of a chromation, or chromate coating, whereby corrosionof the base metal is even further postponed than by mere zinc plating. Aconsiderably better corrosion protection is afforded by thezinc/chromate layer system is than by a mere zinc layer of identicalthickness. Moreover by means of chromation the optical deterioration ofa component due to environmental influences is further postponed—thecorrosion products of zinc, referred to as “white rust”, equallyinterfere with the optical appearance of a component.

The advantages of an applied chromation are so important that almost anygalvanically zinc-plated surface is in addition chromate coated as well.The prior art knows four chromations named after their colorations,which are each applied by treating (immersion, spraying, rolling) azinc-plated surface with the corresponding aqueous chromate coatingsolution. Moreover yellow and green chromations for aluminum are knownwhich are produced analogously. In any case, these are variously thicklayers of substantially amorphous zinc/chromium oxide (oraluminum/chromium oxide) with non-stoichiometric compositions, a certainwater content, and inserted foreign ions. These are known and classifiedinto method groups in accordance with German Industrial Standard (DIN)50960, Part 1:

1) Colorless and Blue Chromations, Groups A and B

The blue chromate layer has a thickness of up to 80 nm, is weakly bluein its inherent color and presents a golden, reddish, bluish, greenishor yellow iridescent coloring brought about by refraction of light inaccordance with layer thicknesses. Very thin chromate layers lackingalmost any inherent color are referred to as colorless chromations(Group A). The chromate coating solution may in either case consist ofhexavalent as well as trivalent chromates and mixtures of both, moreoverconducting salts and mineral acids. There are fluoride-containing andfluoride-free variants. Application of the chromate coating solutions isperformed at room temperature. The corrosion protection of unmarred bluechromations amounts to 10-40 h in the salt spray cabinet according toDIN 50021 SS until the first appearance of corrosion products. Theminimum requirement for Method Groups A and B according to DIN 50961Chapter 10 Table 3 is 8 h for drumware and 16 h for shelfware.

2) Yellow Chromations, Group C

The yellow chromate layer has a thickness of approx. 0.25-1 μm, a goldenyellow coloring, and frequently a strongly red-green iridescentcoloring. The chromate coating solution substantially consists ofhexavalent chromate, conducting salts and mineral acids dissolved inwater. The yellow coloring is caused by the significant proportion(80-220 mg/m²) of hexavalent chromium which is inserted besides thetrivalent chromium produced by reduction in the course of the layerformation reaction. Application of the chromate coating solutions isperformed at room temperature. The corrosion protection of unmarredyellow chromations amounts to 100-200 h in the salt spray cabinetaccording to DIN 50021 SS until the first appearance of corrosionproducts. The minimum requirement for Method Group C according to DIN50961 Chapter 10 Table 3 amounts to 72 h for drumware and 96 h forshelfware.

3) Olive Chromations, Group D

The typical olive chromate layer has a thickness of up to 1.5.μm and isopaquely olive green to olive brown. The chromate coating solutionsubstantially consists of hexavalent chromate, conducting salts andmineral acids dissolved in water, in particular phosphates or phosphoricacid, and may also contain formates. Into the layer considerable amountsof chromium(VI) (300-400 mg/m²) are inserted. Application of thechromate coating solutions is performed at room temperature. Thecorrosion protection of unmarred olive chromations amounts to 200-400 hin the salt spray cabinet according to DIN 50021 SS until the firstappearance of corrosion products. The minimum requirement for MethodGroup D according to DIN 50961 Chapter 10 Table 3 is 72 h for drumwareand 120 h for shelfware.

4) Black Chromations, Group F

The black chromate layer is fundamentally a yellow or olive chromationhaving colloidal silver inserted as a pigment. The chromate coatingsolutions have about the same composition as yellow or olive chromationsand additionally contain silver ions. With a suitable composition of thechromate coating solution on zinc alloy layers such as Zn/Fe, Zn/Ni orZn/Co, iron, nickel or cobalt oxide will be incorporated into thechromate layer as a black pigment so that silver is not required inthese cases. Into the chromate layers considerable amounts ofchromium(VI) are inserted, namely between 80 and 400 mg/m² depending onwhether the basis is a yellow or olive chromation. Application of thechromate coating solutions is performed at room temperature. Thecorrosion protection of unmarred black chromations on zinc amounts to50-150 h in the salt spray cabinet according to DIN 50021 SS until thefirst appearance of corrosion products. The minimum requirement forMethod Group E according to DIN 50961 Chapter 10 Table 3 is 24 h fordrumware and 48 h for shelfware. Black chromations on zinc alloys areconsiderably above the specified values.

5) Green Chromations for Aluminum, Group E

The green chromation on aluminum (known under the name of aluminumgreen) is of a matt green and not iridescent. The chromate coatingsolution substantially consists of hexavalent chromate, conducting saltsand mineral acids dissolved in water as well as particularly phosphatesand silicofluorides. Contrary to a prevailing opinion the formedchromate/phosphate layer is, as evidenced by iodised starch tests, notalways 100% chromium(VI)-free. The production of aluminum green inchromate coating solutions exclusively on the basis of chromium(III) isnot known.

In accordance with the prior art, thick chromate layers affording highcorrosion protection>100 h in the salt spray cabinet according to DIN50021 SS or ASTM B 117-73 until the appearance of first corrosionproducts according to DIN 50961 (June 1987) Chapter 10, in particularChapter 10.2.1.2, in the absence of sealing or any other particularaftertreatment (DIN 50961, Chapter 9) may only be produced by treatmentwith dissolved, markedly toxic chromium(VI) compounds. Accordingly thechromate layers having the named requirements to corrosion protectionstill retain these markedly toxic and carcinogenic chromium(VI)compounds, which are, moreover, not entirely immobilised in the layer.Chromate coating with chromium(VI) compounds is problematic with respectto workplace safety. Use of zinc-plated chromations produced withchromium(VI) compounds, such as the widespread yellow chromations e.g.on screws, constitutes a potential hazard to the population andincreases the general cancer risk.

U.S. Pat. No. 4,384,902, in particular with Examples 1, 2, 4 and 5,describes conversion layers which satisfy the requirements in the saltspray test. In all of the cases, these are cerium-containing layerspresenting a yellowish coloration which is accentuated by the cerium(IV)ion. The examples only contain cerium(III), and hydrogen peroxide as anoxidant, in the bath solution. In the description it is set forth thathydrogen peroxide in the acidic medium does not represent an oxidant forCe(III), however during deposition the pH value nevertheless rises sohigh at the surface that a sufficient amount of Ce(IV) may be generated.The yellowish coloration achieved by this bath composition indeedappears to indicate that an oxidation has taken place—however, only anoxidation from Ce(III) to Ce(IV). Tetravalent cerium is an even morepowerful oxidant than hexavalent chromium, for which reason Ce(IV) willproduce from Cr(III) the Cr(VI) which is to be avoided. Cr(VI) has avery strong yellow coloration and is known as an anticorrosion agent.The layer described in U.S. Pat. No. 4,384,902 is thus not free ofhexavalent chromium.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawings will be provided by the PatentOffice upon request and payment of the necessary fee.

FIG. 1 is a color comparison of various passive layers; it shows acomparison of the present invention with blue and yellow chromations.The substrate is zinc-plated screws. The left picture half is bluechromation; the center is the invention; the right picture half isyellow chromation.

FIG. 2 is a scanning electron microscope image (40,000×) showing acomparison of the present invention (“chromitation”) with blue andyellow chromations. “Gelbchromatierung” means yellow chromation;“Chromitierung” means chromitation; “Blauchromatierung” means bluechromation; “Zink” means zinc.

FIG. 3 is a color photo showing the band width of the iridescentcoloring in accordance with the present invention on zinc surfaces(zinc-plated screws);

FIG. 4 a comparison test with EP 0 034 040, shows coatings of the priorart in accordance with EP 0 034 040. Example 16 is on the left handside, Example 17 is on the right hand side. The upper picture half, onthe outer left and right, shows a black cloth whereby the abrasions onthe metal sheets shown in the top picture half were obtained. Layerportions—discernible as whitish stains—are on both pieces of cloth. Thelower picture half shows the unmarred layers of the prior art. Thesubstrate is zinc-plated steel sheet.

FIGS. 5 to 36 show depth profile analyses of layers according to theinvention and layers resulting from the conventional blue and yellowchromations, wherein the depth profile analyses were measured byglow-discharge spectrometry (spectrometer: JY5000RF);

FIG. 37 is a table containing the evaluation of the depth profileanalyses of FIGS. 5 to 36.

FIG. 38 is a computer simulation of the kinetic model of chromatecoating of zinc for various rate constants.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The layer according to the invention is, however, produced in theabsence of any oxidant and consequently free of hexavalent chromium.This can in particular be seen from the fact that the layer according tothe invention is not yellow.

Even where the yellow coloration and the enhanced corrosion protectionshould be brought about by nothing but Ce(IV), the layer according tothe invention affords the desired corrosion protection even without thisvery costly and rare addition.

U.S. Pat. No. 4,359,348 also describes conversion layers which satisfythe above mentioned requirements in the salt spray test. These, too, inall cases are cerium-containing layers which present the yellowishcoloration accentuated by the cerium(IV) ion. This document thus doesnot exceed U.S. Pat. No. 4,384,902.

It is therefore an object of the present invention to furnish achromium(VI)-free, thick conversion layer having a high chromium contenton zinc or zinc alloys.

For the purposes of the present inventions the applicant coined the term“chromitation” in order to clearly distinguish the present inventionfrom the chromations which are customary in the prior art, and in orderto make clear that neither the obtained conversion layer nor thecompositions (concentrates/passivation baths) whereby the coatingsaccording to the invention are produced contain the toxic chromium(VI),whereas the obtained corrosion protection nevertheless is superior tothat of yellow chromation.

EP 00 34 040 A1 does describe a multiplicity of layers, of the largergroup of which (produced under the standard conditions set forth byBarnes/Ward) the color is not specified, however referred to as clear.Two Examples, namely Nos. 16 and 17, describe a greenishborate-containing layer described as cloudy-dull to non-transparent.

Example 14 describes a layer affording a corrosion protection of only 4hours.

Concerning the features of the invention, the following should be noted:

In glow-discharge spectrometry several elements could not be detectedwhile others could not be calibrated. Therefore thechromium/(chromium+zinc) phases were compared to each other. Thechromium index is the average chromium content in the layer>1% Cr,multiplied by the layer thickness. The chromium index is proportional tothe chromium quantity on the surface (mg/m²).

Further advantages and features of the present invention result from thedescription of embodiments and from theoretical reflections which arenot binding on the one hand and were, on the other hand, carried out bythe inventors while having knowledge of the present invention, and byreferring to the drawings.

The conversion layer preferably has a layer thickness of about 100 to1000 nm, the conversion layer having across the conversion layerthickness a chromium content of greater than 1% based upon zinc andchromium, the conversion layer having an average chromium content ofmore than approximately 5% based on zinc and chromium, and theconversion layer having a chromium index greater than approximately 10,wherein the chromium index is defined as the average chromium content(chromium/(chromium+zinc)) in the layer greater than 1% Cr, multipliedby the layer thickness in nm.

Preferably the conversion layer has a chromium-rich zone greater thanapproximately 20% chromium, based upon zinc and chromium in theconversion layer, of more than approximately 15 nm.

The conversion layer may be transparent, clear, or substantiallycolorless. The conversion layer may be iridescent, and may presentmulti-colored iridescence.

For enhanced corrosion protection the conversion layer may additionallycontain one or more components selected from the group consisting ofsilicate, cerium, aluminum, and borate. The conversion layer may containcobalt or one or more metal compounds having valences of 1 to 6. Theconversion layer may include one or more metal compounds selected fromthe group consisting of Na, Ag, Al, Co, Ni, Fe, Ga, In, La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Zr, Sc, Ti, V, Cr, Mn, Cu,Zn, Y, Nb, Mo, Hf, Ta, and W.

The conversion layer may include one or more ions selected from thegroup consisting of anions and may include one or more ions selectedfrom the group consisting of halide ions, sulfurous ions, nitrate ions,phosphorus-containing ions, carboxylic acid anions, andsilicon-containing anions.

The conversion layer may include one or more ions selected from thegroup consisting of chloride ions, sulfate ions, phosphate ions,diphosphate ions, linear and cyclic oligophosphate ions, linear andcyclic polyphosphate ions, hydrogen phosphate ions, and silicate anions.

The conversion layer may include one or more materials selected from thegroup consisting of polymers, corrosion inhibitors, silicic acids,surfactants, polyols, organic acids, amines, plastics dispersions, dyes,pigments, chromogenic agents, amino acids, siccatives, and dispersingagents.

The conversion layer may include one or more materials selected from thegroup consisting of organic polymers, colloidal or disperse silicicacids, diols, triols, monocarboxylic acids, carbon black, metalchromogenic agents, glycin, and cobalt siccatives.

The conversion layer may include one or more materials selected from thegroup consisting of dyes and color pigments.

In a method according to the invention, a metallic surface preferably istreated with a solution of at least one chromium (III) complex and atleast one salt, wherein chromium (III) is present in the solution in aconcentration of approximately 5 to 100 g/l; and the chromium (III)complex has ligand replacement kinetics more rapid than the fluoridereplacement kinetics in chromium (III)-fluorocomplexes. This methodproduces a chromium (VI)-free conversion layer affording at least thecorrosion protection of conventional chromium (VI)-containing yellowchromations.

Metallic surfaces suitable for application of the conversion layerinclude zinc, zinc alloy, and zinc alloy with iron.

In the method the treating is preferably carried out at an elevatedtemperature, or at a temperature of 20-100° C., more preferably 20-80°C., more preferably 30-60° C., more preferably 40-60° C.

In the method the chromium (III) complex preferably has chelate ligandswhich are selected from the group consisting of dicarboxylic acids,tricarboxylic acids, hydroxycarboxylic acids, acetylacetone, urea, ureaderivatives, mixtures thereof, among each other as well as in mixedcomplexes with inorganic anions and H₂O.

In the method the chromium (III) complex preferably has chelate ligandswhich are selected from the group consisting of oxalic, malonic,succinic, glutaric, adipic, pimelic, suberic, azelaic and sebacic acids,mixtures thereof, and in mixed complexes with inorganic anions and H₂O.

In the method the chromium (III) complex preferably has chelate ligandswhich are selected from the group consisting of maleic acid, phthalicacid, terephthalic acid, tartaric acid, citric acid, malic acid,ascorbic acid, mixtures thereof, and in mixed complexes with inorganicanions and H₂O.

In the method the chromium (III) complex preferably has chelate ligandswhich are selected from the group consisting of malonic acid and malonicacid in mixed complexes with inorganic anions and H₂O.

The method may be performed repeatedly on the metallic surface.

In the method the treating may be carried out at a temperature of 20 to100° C. with rinsing water recycling over at least 2 cascaded rinsingstages; a blue chromation may be performed in one of the rinsing stages.

The method may include an immersion period of between approximately 15and 200 seconds or of between approx. 15 and 100 seconds or an immersionperiod of approx. 30 seconds.

A passivation bath for passivating a metal surface preferably compriseschromium (III) in a concentration of approximately 5 to 100 g/l; thechromium (III) being present in the bath in the form of at least onechromium (III) complex having ligand replacement kinetics more rapidthan the fluoride replacement kinetics in chromium(III)-fluorocomplexes. The bath preferably substantially containschromium (III) as a passivating component.

The chromium (III) complex in the bath preferably is selected fromcomplexes with chromium (III) and at least one chelate ligand selectedfrom the group consisting of dicarboxylic acids, tricarboxylic acids,hydroxycarboxylic acids, acetylacetone, urea, urea derivatives, mixturesthereof, among each other as well as in mixed complexes with inorganicanions and H₂O.

The chromium (III) complex in the bath may be selected from complexeswith chromium(III) and at least one chelate ligand selected from thegroup consisting of oxalic, malonic, succinic, glutaric, adipic,pimelic, suberic, azelaic and sebacic acids, mixtures thereof, and inmixed complexes with inorganic anions and H₂O.

The chromium (III) complex in the bath may be selected from complexeswith chromium(III) and at least one chelate ligand selected from thegroup consisting of maleic acid, phthalic acid, terephthalic acid,tartaric acid, citric acid, malic acid, ascorbic acid, mixtures thereof,and in mixed complexes with inorganic anions and H₂O.

The chromium (III) complex in the bath may be selected from complexeswith chromium (III) and at least one chelate ligand selected from thegroup consisting of malonic acid and malonic acid in mixed complexeswith inorganic anions and H₂O.

The bath may also include one or more components selected from the groupconsisting of sealers, dewatering fluids, additional metal compounds,anions, polymers, corrosion inhibitors, silicic acids, surfactants,polyols, organic acids, amines, plastics dispersions, dyes, pigments,chromogenic agents, amino acids, siccatives and dispersing agents. Thebath may also include one or more components selected from the groupconsisting of 1- to 6-valent metal compounds, halide ions, sulfurousions, nitrate ions, phosphoric ions, carboxylic acid anions,silicon-containing anions, organic polymers, colloidal or dispersesilicic acids, diols, triols, monocarboxylic acids, carbon black,metallic chromogenic agents, glycin, and cobalt siccatives. The bath mayalso include one or more components selected from the group consistingof metal compounds of Na, Ag, Al, Co, Ni, Fe, Ga, In, La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Zr, Sc, Ti, V, Mn, Cu, Zn,Y, Nb, Mo, Hf, Ta, and W, chloride ions, sulfate ions, phosphate ions,diphosphate ions, linear and cyclic oligophosphate ions, linear andcyclic polyphosphate ions, hydrogen phosphate ions and silicate anions.Chromium (III) is preferably present in the bath in a concentration ofapproximately 5 g/l to 80 g/l, more preferably approximately 5 g/l to 60g/l, more preferably approximately 10 g/l to 30 g/l, more preferablyapproximately 20 g/l. The bath preferably has a pH between approximately1.5 and 3, more preferably approximately 2 to 2.5. The temperature ofthe bath is preferably approx. 20 to 100° C., more preferablyapproximately 20 to 80° C., more preferably approximately 30 to 60 ° C.,more preferably approx. 40 to 60° C.

To facilitate preparation of a passivation solution, a concentrate maybe prepared. The concentrate preferably substantially contains chromium(III) for a passivating component, wherein the chromium (III) is presentin the form of at least one complex having ligand replacement kineticsmore rapid than the fluoride replacement kinetics in chromium(III)-fluorocomplexes. The concentrate is preferably in either liquid orsolid form. The concentrate may be used for producing a passivationsolution for passivating a metal surface such as a metal surfaceselected from the group consisting of zinc, cadmium, aluminum and alloysof these metals among each other and/or with iron or other metals.

EXAMPLE 1

The following experiment was carried out:

Small steel parts were bright-zinc coated electrolytically (approx. 15m) and, following galvanisation, singly immersed in a boiling (approx.100° C.), aqueous solution containing:

100 g/l CrCl₃ .6H₂O (trivalent chromium salt) 100 g/l NaNO₃ 15.75 g/lNaF 26.5 g/l citric acid · 1 aq

which had previously been adjusted to a pH value of 2.5 with sodiumhydroxide solution. The immersion time was 30 s. The parts were thenrinsed with water and dried in air flow. On the parts a greenish,strongly iridescent layer had formed which later on turned out to becomprised of zinc/chromium oxide. In the corrosion test in the saltspray cabinet according to DIN 50021 SS it was surprisingly found thatthe chromate layer formed presented a spectacular corrosion protectionuntil the appearance of first corrosion products of 1000 h according toDIN 50961 Chapter 10, in particular Chapter 10.2.1.2.

The novel greenish chromate layer had a layer thickness of approx. 800nm and was produced by a process not involving any chromium(VI) andcould be proven to be chromium(VI)-free.

The production method according to Example 1 for the novel, greenishchromium(VI)-free chromation is not very economical for conventionalplants due to the relatively high temperature of the process solution.Further theoretical reflections concerning chromium(VI)-free chromatecoating and further trials finally resulted in economical productionconditions.

Theoretical Reflections Concerning Chromium(VI)-Free Chromation

Chromate coating of zinc takes place by the formation of a so-calledconversion layer on the zinc surface, i.e. the zinc surface chemicallyreacts with the chromate coating solution and is converted into achromate layer. The formation of conversion layers is a dynamic processbeyond chemical equilibrium. In order to describe the underlyingprocesses, one must therefore employ chemical kinetics. By theespecially established kinetic model it was possible to obtain startingpoints in order to optimise the present invention.

Conversion layer formation in a chromium(III)-based chromate coatingsolution may be described by means of two reaction equations:

I Elementary zinc passes into solution due to acid attack:

II and precipitates on the zinc surface as zinc chromium oxide togetherwith chromium(III):

Zn²⁺+xCr^((III))+yH₂O→ZnCr_(x)O_(y)+2yH⁺

The kinetic model must encompass differential equations for theconcentration developments of Zn², H^(+, Cr) ^((III)) and for thethickness growth of the ZnCrO layer. In the reaction rate startingpoints it was taken into consideration by inserting the term1/(1+p₁·m_(ZnCrO))² that Reaction I is increasingly slowed down by thegrowing passive layer. P1 is a measure for tightness of the layer.$\begin{matrix}{\frac{c_{{Zn}^{2 +}}}{t} =} & {{k_{1} \times {c_{H^{+}}/a}} + {p_{1} \times m_{ZnCrO}\begin{matrix})^{2}\end{matrix}} -} & {{Reaction}\quad I} \\\quad & {{k_{2} \times c_{{Zn}^{2 +}} \times c_{{Cr}^{({III})}}} + {k_{3} \times c_{H^{+}} \times {\tanh \left( {p_{2} \times m_{ZnCrO}} \right)}} +} & {{Reaction}\quad {II}} \\\quad & {k_{T} \times \left( {c_{0,{Zn}^{2 +}} - c_{{Zn}^{2 +}}} \right)} & {{Mass}\quad {transfer}} \\{\frac{c_{H^{+}}}{t} =} & {\left. {{{- 2}k_{1} \times {c_{H^{+}}/a}} + {p_{1} \times m_{ZnCrO}}} \right)^{2} +} & {{Reaction}\quad I} \\\quad & {{2{yk}_{2} \times c_{{Zn}^{2 +}} \times c_{{Cr}^{({III})}}} - {2{yk}_{3} \times 3_{H^{+}} \times {\tanh \left( {p_{2} \times m_{ZnCrO}} \right)}} +} & {{Reaction}\quad {II}} \\\quad & {k_{T} \times \left( {c_{0,H^{+}} - c_{H^{+}}} \right)} & {{Mass}\quad {transfer}} \\{\frac{c_{{Cr}^{({III})}}}{t} =} & {{{- {xk}_{2}} \times c_{{Zn}^{2 +}} \times c_{{Cr}^{({III})}}} + {{xk}_{3} \times c_{H^{+}} \times {\tanh \left( {p_{2} \times m_{ZnCrO}} \right)}} +} & {{Reaction}\quad {II}} \\\quad & {k_{T} \times \left( {c_{0,{Cr}^{({III})}} - c_{{Cr}^{({III})}} - c_{{Cr}^{({III})}}} \right)} & {{Mass}\quad {transfer}} \\{\frac{m_{ZnCrO}}{t} =} & {{k_{2} \times c_{{Zn}^{2 +}} \times c_{{Cr}^{({III})}}} - {k_{3} \times c_{H^{+}} \times {\tanh \left( {p_{2} \times m_{ZnCrO}} \right)}}} & {{Reaction}\quad {II}}\end{matrix}$

The term tan h(p₂·m_(ZnCrO)) represents the indispensable preconditionof reverse reaction II namely the presence of ZnCrO. The tan h functionprovides for a smooth transition from 0 to 1, which may be adjusted withP2. The differential equation system was resolved numerically by meansof a computer. As a result, the layer thickness developments and theconcentration developments over time were obtained. As starting valuesfor time t₀=0 there were employed: $\begin{matrix}{c_{0,{Zn}^{2 +}} = \quad 0} \\{c_{0,H^{+}} = \quad {10^{- 2}{{mol}/l}\quad \left( {{pH}\quad 2} \right)}} \\{c_{0,{Cr}^{({III})}} = \quad {0,5\quad {{mol}/l}}} \\{m_{0,{ZnCrO}} = \quad 0}\end{matrix}$

In FIG. 38 the layer thickness developments for various values of therate constant kj are represented. For good corrosion protection, thepassive layer should have maximum possible thickness and at the sametime compactness.

FIG. 38 shows a computer simulation of the kinetic model for chromatecoating of zinc for various rate constants.

The faster the initial dissolution of zinc (rate constant k₁) is and thefaster the dissolved zinc precipitates with the chromium(III) (rateconstant k₂), the thicker the chromate layer will become. Layer growthis strongly favored by the presence of zinc already dissolved in thebath, which fact resulted from simulations with c_(O,Zn)2+>0. A lower pHvalue favors dissolution of zinc but also brings about increasedredissolution of the layer.

Based on the model, basically two demands may be established forproducing a maximum possible thickness chromate layer. Reaction I andforward reaction II must take place as rapidly as possible, the reversereaction II must remain slow. In this sense, there result the followingstarting points:

Reaction I a pH optimisation b Avoiding carrying over of inhibitors fromthe zinc bath c Addition of oxidants for accelerating zinc dissolution dAcceleration of zinc dissolution by formation of galvanic elements

Forward Reaction II

e The rate constant k₂ should be as high as possible. Chromium(III)complexes generally have slow kinetics. By using suitable ligands itshould be possible to accelerate the reaction rate.

f Upon use of further transition metal cations in the chromate coatingsolution there also result i.a. higher rate constants than for Cr(III).Moreover these transition metal cations may act as catalysts in ligandreplacement on chromium(III).

Reverse Reaction II

g Insertion of poorly redissolvable hydroxides, e.g. nickel, cobaltand/or copper hydroxide.

Serial investigations were carried out. Starting points a and b areknown to the skilled person. Acceleration of zinc dissolution via pointsc and d did also result in thick coatings, however yellowish ones havinga chromium/zinc ratio of 1:4 to 1:3, which only afforded low corrosionprotection. It was found that good corrosion protection values may onlybe obtained at chromium/zinc ratios above 1:2.

A higher chromium/zinc ratio at concurrently thicker chromate layers isobtained when the rate constant k₂ (starting point e) is raised, or theforward reaction II is accelerated. After the inventors of the presentapplication had realised that hot chromium(III) solutions result insurprising passive layers, there are the following possibilities inconnection with the inventors' theoretical reflections:

Raising the temperature of the chromate coating solution and/or of thepartial surface

Raising the chromium(III) concentration in the process solution

Acceleration of ligand replacement kinetics at the chromium(III).

Herefor one should know that chromium(III) in aqueous solutions isessentially present in the form of hexagonal complexes generally havinghigh kinetic stability, and moreover that ligand replacement is the stepdetermining the rate in forward reaction II. By the selection ofsuitable complex ligands, with which the chromium(III) forms kineticallyless stable complexes, k₂ is accordingly increased.

Addition of elements having a catalytic effect on ligand replacementinto the chromate coating solution.

In serial investigations chelate ligands (such as di- and tricarboxylicacids as well as hydroxydi- and hydroxytricarboxylic acids) as suchforming kinetically less stable complexes with chromium(III), whereasthe fluoride complexes are kinetically very stable. When using only suchchelate ligands for complexing the chromium(III) and omitting fluoridein the passivation solution, excellent results were obtained even at atreatment temperature of only 60° C., as is shown by Examples 2 and 3.

EXAMPLE 2

Electrolytically bright-zinc coated (15 m) steel parts were immersed inan aqueous chromate coating solution containing:

50 g/l CrCl₃.6 H₂O (trivalent chromium salt)

100 g/l NaNO₃

31,2 g/l malonic acid

the pH of which had previously been adjusted to 2.0 with sodiumhydroxide solution. The immersion time was 60 s. Following rinsing anddrying there resulted in the salt spray cabinet according to DIN 50021SS a corrosion protection of 250 h until first attack according to DIN50961.

Malonic acid is a ligand enabling more rapid ligand replacement kineticsat the chromium(III) than the fluoride of Example 1. Good corrosionprotection by far exceeding the minimum requirement of DIN 50961 forMethod Group C (yellow chromation) may thus already be achieved at 60°C.

EXAMPLE 3

Electrolytically bright-zinc coated (15 m) steel parts were immersed inan aqueous chromate coating solution consisting of:

50 g/l CrCl₃.6 H₂O (trivalent chromium salt)

3 g/l Co(NO₃)₂

100 g/l NaNO₃

31,2 g/l malonic acid

previously adjusted to pH 2.0 with sodium hydroxide solution. Immersiontime was 60 s. Following rinsing and drying there resulted in the saltspray cabinet according to DIN 50021 SS a corrosion protection of 350 huntil first attack according to DIN 50961.

Cobalt is an element which was capable, in accordance with the modelconcept, of catalysing ligand replacement and moreover reducing reversereaction II owing to insertion of kinetically stable oxides into thechromate layer, so that the chromate layer altogether should becomethicker. In this point, as well, the model concept established for thepresent invention is verified under practical conditions. Corrosionprotection could once more clearly be enhanced in comparison withExample 3 by nothing but the addition of cobalt into the chromatecoating solution.

Novel greenish chromate layers on zinc were produced in analogy withExample 2 at 40, 60, 80 and 100° C. The layer thicknesses of therespective chromate layers were determined by RBS(=Rutherford-Backscattering) testing. In the Table the correspondingcorrosion protection values in hours of salt spray cabinet according toDIN 50021 SS until first attack according to DIN 50961 Chapter 10 areadditionally listed.

J/° C. thickness/nm Corr. Prot./h 40 100 50-60 60 260 220-270 80 400 350450 100 800  800-1200

Depending on the complex ligands used, which is malonate in Examples 2and 3, it is partly possible to achieve even considerably higher layerthicknesses and corrosion protection values. By complex ligandscontaining as the complexing functional group nitrogen, phosphorus orsulfur, (—NR₂, —PR₂ wherein R independently is an organic, in particularaliphatic radical and/or H, and/or —SR, wherein R is an organic, inparticular aliphatic radical or H,), it is possible to even produce theindicated layer properties within limits at room temperatures.

EXAMPLE 4

Steel parts electrolytically coated with a zinc/iron alloy (0.4-0.6%iron) were immersed at 60° C. in the following aqueous chromate coatingsolution:

50 g/l CrCl₃.6 H₂O

100 g/l NaNO₃

31.2 g/l malonic acid

The solution was beforehand adjusted to pH 2.0 with NaOH. Immersion timewas 60 s. Following rinsing and drying a transparent, greenish, slightlygrey, strongly iridescent layer was visible on the zinc/iron. In thesalt spray cabinet in accordance with the above specified DIN and ASTMstandards there resulted a corrosion protection of 360 h until firstattack according to DIN 50961.

EXAMPLE 5

Steel parts electrolytically coated with a zinc/nickel alloy (8-13%nickel) were immersed at 60° C. into the following aqueous chromatecoating solution:

50 g/l CrCl₃.6 H₂O

100 g/l NaNO₃

31.2 g/l malonic acid

The solution was beforehand adjusted to pH 2.0 with NaOH. Immersion timewas 60 s. Following rinsing and drying a transparent, greenish,dark-grey, strongly iridescent layer was visible on the zinc/nickel. Inthe salt spray cabinet in accordance with the above specified DIN andASTM standards there resulted a corrosion protection of 504 h untilfirst attack according to DIN 50961.

The novel greenish chromium(VI)-free chromate layer accordinglydepending on the production temperature has a thickness of between 100and 1000 nm, has a weakly green inherent color and a red-greeniridescent coloring. The chromate coating solution consists of trivalentchromates, moreover of conducting salts and mineral acids. Applicationof the chromate coating solutions is generally performed at temperaturesabove 40° C. The corrosion protection of unmarred greenishchromium(VI)-free chromate coatings depending on the productiontemperature amounts to 100-1200 h in the salt spray cabinet according toDIN 50021 SS until the first appearance of corrosion products. Thus thenovel chromation satisfies the minimum requirements to corrosionprotection for Method Groups C and D according to DIN 50961 (Chapter 10,Table 3), i.e. without chromium(VI) either in production or in theproduct.

By the present invention it is for the first time made possible toprovide chromium(VI)-free conversion layers or passive layers on thebasis of chromium(III), which do, however, furnish the corrosionprotection of yellow chromations customary in the prior art—i.e., ofchromium(VI)-containing passive layers.

This is a singular novelty in the entire galvanisation industry.

Hitherto on a chromium(III) basis only clear to blue layers, referred toas “blue passivation” in technical circles, were known which arevariously applied practically.

Moreover yellowish-transparent layers with an addition of cerium areknown which are, however, not used practically owing to the very costlycerium addition and their poor corrosion protection properties.

Moreover powdery-greenish layers are known for which the applicant—oneof the leading enterprises in the field of surface technology—is notaware of any practical applications.

Even the difference in terms of color of the conversion layers of thepresent invention is conspicuous in FIG. 1, wherein three treatmentmethods were performed on zinc-plated screws.

The left-hand pile of screws in accordance with the illustration of FIG.1 was subjected to a classical blue chromation in accordance with thestandard of Method Group B according to DIN 50961 Chapter 10 table 3.

The right-hand pile of screws on the photograph according to FIG. 1 wassubjected to a conventional yellow chromation in accordance with thestandard of Method Group C according to DIN 50961 Chapter 10 table 3.

The center pile of screws shows the result of passivation of the screwsby means of the method in accordance with the invention.

This is consequently a greenish-iridescent, transparent conversionlayer, or passive layer.

Moreover the colors represented in FIG. 1 are the true colors, which canbe seen from the fact that a color plate and moreover a grey wedge wasjointly photographed for the purpose of neutral color representation.

As can be seen from the white test field “White” and from thecorresponding field having the density “0.00” from the grey wedge, bothtest fields are pure white, making evident the neutral filtering and theresulting realistic color representation.

In FIG. 2 scanning electron microscope (SEM) images of the conversionlayers of a yellow chromation and of a blue chromation in accordancewith the prior art are shown in comparison with the “chromitation” ofthe present invention.

The layer samples are derived from the correspondingly passivatedzinc-plated iron screws shown in FIG. 2, lower half.

The samples treated in accordance with the invention (by “chromitation”)presented a chromium(VI)-free conversion layer having a thickness ofapprox. 300 nm. In the photographs of FIG. 2 it should be consideredthat the layers were photographed in a viewing angle of approx. 40°,resulting in foreshortening by approx. cos (40°)=0.77.

Based on the SEM images of the chromitation layer of the invention ittherefore results that conversion layer thicknesses like in yellowchromation are obtained, however with the difference that the conversionlayer of the invention does not contain any toxic chromium(VI).

The color photograph of FIG. 3 moreover shows the bandwidth of theiridescent coloring of the passive layer according to the inventionunder practical conditions.

It can already be seen in the photographs of FIGS. 1 and 3 that thepassive layer according to the invention does not contain anychromium(VI) ions as it lacks the typically yellow color (cf. right-handpile of screws of the color photograph of FIG. 1).

Objects according to the photograph of FIGS. 1 and 3 as well aszinc-plated steel sheets passivated by the method of the invention weretested in the salt spray cabinet according to DIN50021SS or ASTM B117-73, respectively, until the occurrence of first corrosion productsaccording to DIN50961 Chapter 10. Herein it was surprisingly found thatthe passive layers of the present invention, and thus the objectspassivated by the present method, fulfilled the corrosion protection ofchromium(VI) passivations, i.e. yellow chromations, although notcontaining any chromium(VI).

It is worth mentioning that a typical yellow chromation of the prior artaffords resistance for approx. 100 hours of exposure to saltwater inaccordance with the above specified DIN or ASTN standard, whereas eventhe tenfold corrosion protection was achieved by the passive layers ofthe present invention.

The layers of the present invention as well as the methods for producingthis layer, or the method for passivation of metal surfaces, thussatisfy the long-standing demand in this technical field for conversionlayers doing without any toxic and carcinogenic chromium(VI) compoundswhile nevertheless even presenting and generally even excelling thecorrosion protection of yellow chromations.

EP 00 34 040 A1 does describe a multitude of layers, wherein thecolorations of the larger group thereof (produced under the standardconditions set forth by Barnes/Ward) are not specified, however whichare referred to as clear. Two examples, i.e. Nos. 16 and 17, describe agreenish, borate-containing layer referred to as cloudy-dull tonon-transparent.

Example 14 describes a layer affording a corrosion protection of no morethan 4 hours.

In Example 15 of EP 00 34 040, an aluminum-containing layer is describedwhich attains a corrosion protection of 100 hours. This is achieved—incomparison with the remaining examples—merely by the corrosionprotection additive aluminum which is lacking in the present invention.Aluminum-free layers of identical or similar baths do, however, onlypresent poor corrosion protection. The layer according to the inventionoffers significantly higher corrosion protection, namely up to 1000 h,even without this addition.

Examples 16 and 17 describe layers affording a corrosion protection of300 and 200 hours in the salt spray test and thus in the range claimedby the applicant. Description page 19, line 7 sets forth that layers ofmore than 1000 nm are required for good corrosion protection. It is thusunderstandable that these layers, without exception moreover producedfrom solutions containing boric acid, are described to be cloudy andrather non-transparent (page 14, line 10). The enhanced corrosionprotection, in accordance with page 15, lines 1-5, is due to theinsertion of borate-containing species.

The layer according to the invention, on the other hand, also offershigh (and even higher) corrosion protection without this addition.

There is, however, another difference that is relevant in terms ofpatent law as well as in practical application: namely, the layersdescribed in Examples 16 and 17 of EP 00 34 040 are soft and come offwhen wiped and consequently require some sort of hardening process as anaftertreatment (page 17, lines 12-21).

The present layers according to the invention are hard and resistant towiping even without a hardening process, and adhere well. Corrosionprotection layers which come off when wiped and which do not adhere tothe substrate are useless for practical application.

In FIG. 4, a photograph is shown as a comparison example. Thisphotograph represents the result of comparison tests carried out by theapplicant in comparison with EP 00 34 040. In particular the applicantreproduced the Examples 16 and 17 given in this prior art. Herein steelsheets were immersed into the solutions described in Examples 16 and 17of EP 00 34 040 and the respective treatment times were observed. FIG. 4shows the layers on the substrate surfaces obtained in accordance withthe prior art, namely from the top to the bottom the first and secondsheets successively treated by immersion.

The photograph of FIG. 4 shows from the left to the right in the tophalf of the illustration a cloth whereby the layer produced inaccordance with Example 16—prior art—was wiped, a zinc-plated steelsheet treated in accordance with Example 16, beside it a zinc-platedsteel sheet treated in accordance with Example 17—prior art—and on theextreme right also a cloth whereby the layer of Example 17 was wiped. Inthe second line on the left side—beside the indication of Example 16 andbeside it to the right (beside the indication of Example 17) arespective zinc-plated steel sheet coated in accordance with the priorart is shown.

What is visible is a milky, white-greenish powdery coating which alreadycomes off when wiped with a soft cloth even without application ofparticular pressure (see FIG. 4, top half of illustration). The priorart itself suggests that this layer is not a compact oxidezinc-/chromium conversion layer firmly adhering to the substrate sheetbut a loosely overlying coating substantially consisting of chromiumhydroxide. The pH for this coating must be so high that theprecipitation limit for chromium hydroxides is already exceeded (page26, line 12 of EP 0034 040). Precipitation of chromium hydroxide iskinetically inihibited and is favored by immersion of a more or lessrough surface. The fact that the layer formation mechanism has to be adifferent one from the other examples may also be seen from thecircumstance that with (Example 16 prior art) or without (Example 17)complexing agents more or less the same result was achieved. Inpractical reproduction of Examples 16 and 17 of the prior art it wasmoreover found that the layer became thicker, softer and more powderywith an increasing number of metal sheets coated in the solution. Inaddition, more and more chromium hydroxide precipitated, whereby theuseful life of such a coating solution is limited to a few hours. Thelayer according to the invention, on the other hand, is produced onlyfrom suitable “rapid” complexes and furthermore in a distinctly acidicpH range. The solution is stable over months, presumably even years.

The measurements underlying FIGS. 5 to 36 were performed with aglow-discharge spectrometer.

The element F and die anions could not be analysed by this method. O H,Cl and K could not be quantified.

The following Table shows the concentration ranges for which calibrationis valid:

Element Concentration min. in Concentration max. in C 0.0067 3.48 S0.0055 0.168 Cr 0.0001 99.99 Ni 0.0001 99.99 Co 0.0001 7.00 Zn 0.000199.99 Na 0.0001 0.0068 N 0.0001 6.90 B 0.0001 0.040 Fe 0.0005 99.91

Sample allocation in FIGS. 5 to 36 results from the following Table:

Sample Measurement No. Coating Conditions point 1 Chromitation on 60°C., 1 min, pH 2 A Zn (invention) B 2 60° C., 2 min, pH 2 A B 3 60° C., 1min, pH 2.5 A 4 60° C., 1.5 min, pH A 2.5 5 60° C., 2 min, pH 2.5 A 6100° C., 1 min, pH 2 A B C D 7 Chromitation on 60° C., 1 min, pH 2 AZn/Fe B 8 Blue chromation 20° C., 30s, pH 1.8 A on Zn 9 Yellowchromation 20° C., 45s, pH 1.8 A on Zn B

FIG. 37 shows a Table containing the evaluations of the depth profilemeasurements, which indicates that all of the (chromitation) layers ofthe invention have thicknesses exceeding 100 nm.

What is claimed is:
 1. A conversion layer comprising chromium(III), saidconversion layer being chromium(VI)-free, said conversion layer being asubstantially coherent conversion layer on zinc or a zinc alloy, whereineven in the absence of silicate, cerium, aluminum and borate saidconversion layer presents a corrosion protection of about 100 to 1000 hin the salt spray test according to DIN 50021 SS or ASTM B 117-73 untilfirst attack according to DIN 50961 Chapter 10, said conversion layerhaving a layer thickness of about 100 nm to 1000 nm, said conversionlayer having across the conversion layer thickness a chromium content ofgreater than 1% based upon zinc and chromium, said conversion layerhaving an average chromium content of more than approximately 5% basedon zinc and chromium, said conversion layer having a chromium indexgreater than approximately 10, wherein the chromium index is defined assaid average chromium content (chromium/(chromium+zinc)) in the layergreater than 1% Cr, multiplied by the layer thickness in nm.
 2. Aconversion layer according to claim 1, wherein said conversion layer hasa chromium-rich zone greater than approximately 20% chromium, based uponzinc and chromium in the conversion layer, of more than approximately 15nm.
 3. A conversion layer according to claim 1, wherein said layer istransparent.
 4. A conversion layer according to claim 1, wherein saidlayer is clear.
 5. A conversion layer according to claim 1, wherein saidlayer is substantially colorless.
 6. A conversion layer according toclaim 1, wherein said layer is iridescent.
 7. A conversion layeraccording to claim 1, wherein said layer presents multi-colorediridescence.
 8. A conversion layer according to claim 1, wherein saidlayer is hard.
 9. A conversion layer according to claim 1, wherein saidlayer is resistant to wiping.
 10. A conversion layer according to claim1, wherein said layer adheres well.
 11. A conversion layer according toclaim 1, wherein said layer contains, for further enhanced corrosionprotection, one or more components selected from the group consisting ofsilicate, cerium, aluminum and borate.
 12. A conversion layer accordingto claim 1, wherein said layer further comprises cobalt.
 13. Aconversion layer according to claim 1, wherein said layer furthercomprises one or more metal compounds selected from the group consistingof 1- to 6-valent metal compounds.
 14. A conversion layer according toclaim 1, wherein said layer further comprises one or more metalcompounds selected from the group consisting of Na, Ag, Al, Co, Ni, Fe,Ga, In, Lanthanides, Zr, Sc, Ti, V, Cr, Mn, Cu, Zn, Y, Nb, Mo, Hf, Taand W.
 15. A conversion layer according to claim 1, wherein said layerfurther comprises one or more ions selected from the group consisting ofanions.
 16. A conversion layer according to claim 1, wherein said layerfurther comprises one or more ions selected from the group consisting ofhalide ions, sulfurous ions, nitrate ions, phosphorus-containing ions,carboxylic acid anions and silicon-containing anions.
 17. A conversionlayer according to claim 1, wherein said layer further comprises one ormore ions selected from the group consisting of chloride ions, sulfateions, phosphate ions, diphosphate ions, linear and cyclic oligophosphateions, linear and cyclic polyphosphate ions, hydrogen phosphate ions, andsilicate anions.
 18. A conversion layer according to claim 1, whereinsaid layer further comprises one or more materials selected from thegroup consisting of polymers, corrosion inhibitors, silicic acids,surfactants, polyols, organic acids, amines, plastics dispersions, dyes,pigments, chromogenic agents, amino acids, siccatives, and dispersingagents.
 19. A conversion layer according to claim 1, wherein said layerfurther comprises one or more materials selected from the groupconsisting of organic polymers, colloidal or disperse silicic acids,diols, triols, monocarboxylic acids, carbon black, metal chromogenicagents, glycin, and cobalt siccatives.
 20. A conversion layer accordingto claim 1, wherein said layer further comprises one or more materialsselected from the group consisting of dyes and color pigments.
 21. Amethod for producing a chromium(VI)-free conversion layer affording atleast the corrosion protection of conventional chromium(VI)-containingyellow chromations, said method comprising the step of treating ametallic surface with a solution of at least one chromium(III) complexand at least one salt, wherein chromium(III) is present in said solutionin a concentration of approx. 5 to 100 g/l; and said chromium(III)complex has ligand replacement kinetics more rapid than the fluoridereplacement kinetics in chromium(III)-fluorocomplexes, said methodproducing a chromium(VI)-free conversion layer affording at least thecorrosion protection of conventional chromium(VI)-containing yellowchromations.
 22. A method according to claim 21, wherein said metallicsurface is zinc or zinc alloy.
 23. A method according to claim 21,wherein said metallic surface is zinc or zinc alloy with iron.
 24. Amethod according to claim 21, wherein said treating is carried out at anelevated temperature.
 25. A method according to claim 21, wherein saidtreating is carried out at a temperature of 20 to 100° C.
 26. A methodaccording to claim 21, wherein said treating is carried out at atemperature of 20 to 80° C.
 27. A method according to claim 21, whereinsaid treating is carried out at a temperature of 30 to 60° C.
 28. Amethod according to claim 21, wherein said treating is carried out at atemperature of 40 to 60° C.
 29. A method according to claim 21, whereinsaid chromium(III) complex has chelate ligands which are selected fromthe group consisting of dicarboxylic acids, tricarboxylic acids,hydroxycarboxylic acids, acetylacetone, urea, urea derivatives, mixturesthereof, among each other as well as in mixed complexes with inorganicanions and H₂O.
 30. A method according to claim 21, wherein saidchromium(III) complex has chelate ligands which are selected from thegroup consisting of oxalic, malonic, succinic, glutaric, adipic,pimelic, suberic, azelaic and sebacic acids, mixtures thereof, and inmixed complexes with inorganic anions and H₂O.
 31. A method according toclaim 21, wherein said chromium(III) complex has chelate ligands whichare selected from the group consisting of maleic acid, phthalic acid,terephthalic acid, tartaric acid, citric acid, malic acid, ascorbicacid, mixtures thereof, and in mixed complexes with inorganic anions andH₂O.
 32. A method according to claim 21, wherein said chromium(III)complex has chelate ligands which are selected from the group consistingof malonic acid and malonic acid in mixed complexes with inorganicanions and H₂O.
 33. A method according to claim 21, wherein said methodis performed repeatedly on said metallic surface.
 34. A method accordingto claim 21, wherein said treating is carried out at a temperature of 20to 100° C. with rinsing water recycling over at least 2 cascaded rinsingstages.
 35. A method according to claim 34, wherein a blue chromation isperformed in one of the rinsing stages.
 36. A method according to claim21, wherein said method includes an immersion period of between approx.15 and 200 seconds.
 37. A method according to claim 21, wherein saidmethod includes an immersion period of between approx. 15 and 100seconds.
 38. A method according to claim 21, wherein said methodincludes an immersion period of approx. 30 seconds.
 39. A passivationbath for passivating a metal surface, said bath comprising chromium(III)in a concentration of approx. 5 to 100 g/l, chromium(III) being presentin said bath in the form of at least one chromium(III) complex havingligand replacement kinetics more rapid than the fluoride replacementkinetics in chromium(III)-fluorocomplexes, said bath substantiallycontaining chromium(III) as a passivating component.
 40. A passivationbath according to claim 39, wherein said metal surface is zinc or zincalloy.
 41. A passivation bath according to claim 39, wherein saidchromium(III) complex is selected from complexes with chromium(III) andat least one chelate ligand selected from the group consisting ofdicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids,acetylacetone, urea, urea derivatives, mixtures thereof, among eachother as well as in mixed complexes with inorganic anions and H₂O.
 42. Apassivation bath according to claim 39, wherein said chromium(III)complex is selected from complexes with chromium(III) and at least onechelate ligand selected from the group consisting of oxalic, malonic,succinic, glutaric, adipic, pimelic, suberic, azelaic and sebacic acids,mixtures thereof, and in mixed complexes with inorganic anions and H₂O.43. A passivation bath according to claim 39, wherein said chromium(III)complex is selected from complexes with chromium(III) and at least onechelate ligand selected from the group consisting of maleic acid,phthalic acid, terephthalic acid, tartaric acid, citric acid, malicacid, ascorbic acid, mixtures thereof, and in mixed complexes withinorganic anions and H₂O.
 44. A passivation bath according to claim 39,wherein said chromium(III) complex is selected from complexes withchromium(III) and at least one chelate ligand selected from the groupconsisting of malonic acid and malonic acid in mixed complexes withinorganic anions and H₂O.
 45. A passivation bath according to claim 39,wherein said bath further comprises one or more components selected fromthe group consisting of sealers, dewatering fluids, additional metalcompounds, anions, polymers, corrosion inhibitors, silicic acids,surfactants, polyols, organic acids, amines, plastics dispersions, dyes,pigments, chromogenic agents, amino acids, siccatives and dispersingagents.
 46. A passivation bath according to claim 39, wherein said bathfurther comprises one or more components selected from the groupconsisting of 1- to 6-valent metal compounds, halide ions, sulfurousions, nitrate ions, phosphoric ions, carboxylic acid anions,silicon-containing anions, organic polymers, colloidal or dispersesilicilic acids, diols, triols, monocarboxylic acids, carbon black,metallic chromogenic agents, glycin, and cobalt siccatives.
 47. Apassivation bath according to claim 39, wherein said bath furthercomprises one or more components selected from the group consisting ofmetal compounds of Na, Ag, Al, Co, Ni, Fe, Ga, In, Lanthanides, Zr, Sc,Ti, V, Mn, Cu, Zn, Y, Nb, Mo, Hf, Ta and W, chloride ions, sulfate ions,phosphate ions, diphosphate ions, linear and cyclic oligophosphate ions,linear and cyclic polyphosphate ions, hydrogen phosphate ions andsilicate anions.
 48. A passivation bath according to claim 39, whereinchromium(III) is present in a concentration of approx. 5 g/l to 80 g/l.49. A passivation bath according to claim 39, wherein chromium(III) ispresent in a concentration of approx. 5 g/l to 60 g/l.
 50. A passivationbath according to claim 39, wherein chromium(III) is present in aconcentration of approx. 10 g/l to 30 g/l.
 51. A passivation bathaccording to claim 39, wherein chromium(III) is present in aconcentration of approx. 20 g/l.
 52. A passivation bath according toclaim 39, wherein said bath has a pH between approx. 1.5 and
 3. 53. Apassivation bath according to claim 39, wherein said bath containsapprox. 20 g/l chromium(III) and has a pH of approx. 2 to 2.5.
 54. Apassivation bath according to claim 39, wherein said bath has a bathtemperature of approx. 20 to 100° C.
 55. A passivation bath according toclaim 39, wherein said bath has a bath temperature of approx. 20 to 80°C.
 56. A passivation bath according to claim 39, wherein said bath has abath temperature of approx. 30 to 60° C.
 57. A passivation bathaccording to claim 39, wherein said bath has a bath temperature ofapprox. 40 to 60° C.
 58. A concentrate for producing a passivationsolution for passivating a metal surface, said concentrate substantiallycontaining chromium(III) for a passivating component, wherein saidchromium(III) is present in the form of at least one complex havingligand replacement kinetics more rapid than the fluoride replacementkinetics in chromium(III)-fluorocomplexes.
 59. A concentrate accordingto claim 58, wherein said concentrate is present in liquid form.
 60. Aconcentrate according to claim 58, wherein said concentrate is presentin solid form.
 61. A concentrate according to claim 58, wherein saidmetal surface is zinc or zinc alloy.
 62. A concentrate according toclaim 58, wherein said metal surface is selected from the groupconsisting of zinc, cadmium, aluminum and alloys of these metals amongeach other and/or with iron or other metals.